Hi Tidyman
You said you had a snail trail that you cleaned up? Often the trail is symptom of a microscopic crack that is shorting some high voltage to ground. When the plug is under compression it takes a lot more energy to fire the plug, than when you look at a test plug outside the engine. Another thing is to check the injector at the missing cylinder with a noid light, to make sure the signal is getting to the injector. It also could be a faulty injector failing when it warms up. The valves or lifters Would most likely fail on startup and then become better as it warms up.